how do you know if a wine will age well?

If it's inexpensive it likely won't improve, but that doesn't mean you can't keep it for a while as long it it's properly stored.

Will appreciate a more comprehensive explanation of the topic, given that sometimes price and quality are not always uniform.

It will last a couple of years and over time the fruit will diminish. Unlike some other varietals, the underlyin flavors that remain just aren't very exciting.

Here is a statement about zinfandel's potential for aging from Wine Spectator:

"Should you try aging Zinfandel in your cellar? In theory, there's no reason why Zinfandel shouldn't age well. In my experience, a few Zins do, but I've rarely found them more interesting, flavorful or complex at ages five, seven or 10 than I did at ages one or two. One of the biggest trade-offs in aging Zin is that, with time, the distinctive wild berry and raspberry flavor often disappears and is replaced by a tarry, earthy, sagelike quality. I hate the trade-off because that raspberry flavor is so wonderful. "

Dimitri